Patients with sickle cell disease are immunocomproised; they acquire functioal asplenia early in life, have diminished synthesis of type specific antibody, defective opsonization and, perhaps, an abnormality in the alternate complement pathway (20). These patients are also frequent rrecipients of cellular blood products (39,50). We have made preliminary observations with the Children's Hospital sickle cell disease (SCD) population, using quantitive allele-specific PCR (28-30) for the detection of male donor cells in the circulation of female recipients. We have found that 6 of 16 SCD patients appear to have persistent circulating donor WBC's 3 weeks to 2 years following transfusion. Since all CHO patients have received leukoreduced blood products since at least the late 1980's, this was an especially surprising finding. We have formed 2 hypotheses related to the transfusion of these products in SCD patients: 1) Existence of chimerism: The combination of native immune deficit in SCD and recurring exposure to allogeneic mononuclear cells through transfusion may favor the persistence of donor white blood cells (WBC's) beyond 30 days in some patients, resulting in a state of microchimerism. 2) Associated clinical factors: The possibility for development of a chimeric state following transfusion may be related to one or more of the following: age of the patient, number of previous transfusions, presence of HLA alloantibodies (from prior transfusion), time interval since last transfusion, characteristics of the blood product (number and viability of transfused WBC's), degree of histocompability between donor and recipient.